This application is based upon and claims priority of Japanese Patent Application No. 2000-162902, filed on May 31, 2000, the contents being incorporated herein by reference.
1. Field of the Invention
The present invention relates to boosting methods and apparatus, in particular, suitably used for semiconductor integrated circuits including circuits for boosting power supply voltages.
2. Description of the Related Art
Conventionally, in a device such as a semiconductor integrated circuit, when a voltage higher than an external power supply voltage is required, a boosting circuit such as a charge pump in the device is used to boost the external power supply voltage to the voltage required by the device. This boosted voltage depends on the level of the external power supply voltage. Therefore, if the external power supply voltage is slightly lower than the boosted voltage required by the device, the external power supply voltage is boosted twice by using one charge pump. If the boosted voltage required by the device cannot be obtained by twofold boosting, the external power supply voltage is boosted by three times by using two charge pumps.
In designing a device using the voltage obtained by boosting an external power supply voltage with boosting circuits such as charge pumps, it is determined in the early designing stage whether the number of boosting stages is one or two.
However, if the device is designed after the number of boosting stages is determined in the early designing stage, an operation error may occur in the device after it is manufactured. Assume that the boosted voltage required by the device is almost equal to the voltage obtained by one-stage boosting. In this case, if the device is designed as a one-stage boosting circuit, the device sometimes does not operate due to a shortage of voltage. In such a case, the device must be redesigned from the beginning, which requires much labor.
In one method for solving this problem, two charge pumps are switched so that the two charge pumps are used when the boosted voltage to be used by a device is higher than an external power supply voltage, and only one charge pump is used when the boosted voltage is slightly higher than the power supply voltage. For example, according to the driving voltage supplying apparatus disclosed in Japanese Patent Application Laid-Open No. 6-78527, two charge pumps are set up in an integrated circuit. When only one stage of boosting is required, only one charge pump predetermined is operated while the other charge pump is not operated. When two stages of boosting are required, both the charge pumps are operated.
In this method in which two charge pumps are set up in a device and one or two charge pumps are selectively used, however, when the voltage required by the device can be obtained by only one stage of boosting, the other charge pump becomes a redundant circuit because it is not operated. Besides, since the pumping capacitor of each charge pump has a large circuit area, a problem arises in terms of a waste of a circuit area particularly in a high-density circuit such as an integrated circuit.
It is an object of the present invention to provide boosting methods and apparatus wherein the numbers of boosting stages can be changed even after being manufactured, without wasting any circuit area.
A boosting apparatus according to the present invention for boosting a power supply voltage, is characterized in that a plurality of cascade-connected boosting circuits are controlled to boost voltages at the same timing when a boosting mode signal output from a selection circuit indicates a first mode.
A boosting method according to the present invention using a boosting apparatus including a plurality of cascade-connected boosting circuits, is characterized in that the plurality of boosting circuits are controlled to boost voltages at the same timing in response to a boosting mode signal.
The present invention having the above construction includes a plurality of cascade-connected boosting circuits, and a control circuit controls the plurality of boosting circuits in accordance with a boosting mode signal output from a selection circuit. If the boosting mode signal output from the selection circuit indicates a first mode, the plurality of boosting circuits are controlled to boost power supply voltages at the same timing. This makes it possible to change the number of boosting stages. In addition, in the first mode, the plurality of boosting circuits operate concurrently as a single boosting circuit to boost the power supply voltage. Therefore, even if the boosting modes are switched, the plurality of boosting circuits can be effectively used without wasting any circuit area.
In addition, since the selection circuit for outputting a boosting mode signal is provided, the optimum number of boosting stages can be selected at an arbitrary point of time if a problem such as a shortage of voltage is detected not only before but even after the apparatus is manufactured.